Atomistic Insights into the Conversion Reaction in Iron Fluoride: A Dynamically Adaptive Force Field Approach

Metal Fluoride Cathode Materials for Lithium Rechargeable Batteries: Focus on Iron Fluorides

Exploring prospective rechargeable batteries with high energy densities is urgently needed on a worldwide scale to address the needs of the large-scale electric vehicle market. Conversion-type metal fluorides (MFs) are attractive cathodes for next-generation rechargeable batteries because of their high theoretical potential and capacities and provide new perspectives for developing novel battery systems that satisfy energy density requirements. However, some critical issues, such as high voltage hysteresis and poor cycling stability must be solved to further enhance MF cathode materials. In this review, the recent advances in mechanisms focused on FeF₃ cathodes under lithiation/delithiation processes are discussed in detail. Then, the classifications and advantages of various synthesis methods to prepare MF-based materials are first minutely discussed. Moreover, the performance attenuation mechanisms of MFs and the effort in the development of mitigation strategies are comprehensively reviewed. Finally, prospects for the current obstacles and possible research directions, with the aim to provide some inspiration for the development of MF cathode-based batteries are presented.

Predictive Molecular Models for Charged Materials Systems: From Energy Materials to Biomacromolecules

Electrostatic interactions play a dominant role in charged materials systems. Understanding the complex correlation between macroscopic properties with microscopic structures is of critical importance to develop rational design strategies for advanced materials. But the complexity of this challenging task is augmented by interfaces present in the charged materials systems, such as electrode-electrolyte interfaces or biological membranes. Over the last decades, predictive molecular simulations that are founded in fundamental physics and optimized for charged interfacial systems have proven their value in providing molecular understanding of physicochemical properties and functional mechanisms for diverse materials. Novel design strategies utilizing predictive models have been suggested as promising route for the rational design of materials with tailored properties. Here, an overview of recent advances in the understanding of charged interfacial systems aided by predictive molecular simulations is presented. Focusing on three types of charged interfaces found in energy materials and biomacromolecules, how the molecular models characterize ion structure, charge transport, morphology relation to the environment, and the thermodynamics/kinetics of molecular binding at the interfaces is discussed. The critical analysis brings two prominent field of energy materials and biological science under common perspective, to stimulate crossover in both research field that have been largely separated.

Enabling Long Cycle Life and High Rate Iron Difluoride Based Lithium Batteries by In Situ Cathode Surface Modification

Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF₂ and polymer-derived carbon (FeF₂ @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF₂ embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF₂ @PDC enables a reversible capacity of 500 mAh g^-1 with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF₂ @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g^-1 after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe₃ O₄ layers on the surface of FeF₂ prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF₂ electrochemistry, and a strategy to radically improve the electrochemical performance of FeF₂ cathode for lithium-ion battery applications.

Computational mechanistic insights on Ag2O as a host for Li in lithium-ion batteries

The lithiation mechanism of Ag₂O with one and two Li atoms per unit cell carried out using density functional theory (DFT) studies indicate the adsorption of one Li atom at the tetrahedral interstitial site (TIS) as the most stable one with the formation of the Li-Ag₂O compound. PDOS plots depict the hybridization of Li 2s with O 2p and Ag 5s states, resulting in the formation of both Li-O and Li-Ag bonds. Also, the Bader charge analysis validates the bonding of Li at TIS with both Ag and O atoms. The band structure plots showcase a surge in electronic conductivity, accounting for the metallic transition of Ag₂O with the addition of Li. The difference in the charge distribution between the pairs of surface and subsurface silver atoms in charge density difference plots (CDDP) reveal the incorporation of Li, invoking charge inequivalence in addition to the symmetry inequivalence in the optimized structure. In the case of two Li atoms, Li placed on the top of oxygen (O_top) and at the subsurface of silver (Ag_ss) was found to be the most stable structure with the formation of two strong Li-O bonds, indicating the feasibility of the conversion mechanism. The increase in the Bader charge of O (-1.28e) as well as CDDP establish the formation of Li₂O by the conversion mechanism. Thus, the increase in the Li⁺ ion concentration becomes the deciding factor for the transition of the lithiation mechanism from Li-Ag₂O compound formation to the conversion mechanism. The molecular dynamics (MD) simulations feature Li as an "antisite defect producing guest", leading to the formation of Li-O bonds with a concomitant reduction in the number of Ag-O bonds as a function of time. This confirms a shift in the reaction kinetics from the formation of Li-Ag₂O to the conversion mechanism.

Micrometer-Sized, Dual-Conductive MoO2 /β-MoO3- x Mosaics for High Volumetric Capacity Li/Na-Ion Batteries

The transition metal oxides (TMOs) with high volumetric capacities are promising anodes for the future electronics, however, they usually suffer from severe capacity decay and poor rate capability. Carbon hybridization and nanosizing can resolve these challenges, yet these significantly compromise the volumetric capacity. Herein, both high capacity and long cycling stability are simultaneously achieved in the micrometer-sized Mo-based oxide particles by designing the dual conductive MoO₂ /β-MoO_{3- x} mosaics. The rational combination of the highly electronically conductive MoO₂ with the highly ionically conductive and open-structured β-MoO₃ achieves a promising volumetric capacity of 1742 mAh cm^-3 , which is four times higher than the commercial graphite. Simultaneously, both stable cycling performance (87% retention after 500 cycles) and excellent rate capability (outperformed a majority of the MoO₂ -based anodes reported in literature) are obtained in the lithium-ion batteries. For the sodium-ion batteries, the composite exhibits three times higher Na⁺ storage than pure MoO₂ . Moreover, the decisive role of the bond energy on the electrochemical performance of TMOs is also identified. This study may open up new perspectives for choosing and designing the TMO anodes with a high volumetric capacity for the practical applications.

Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes

The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(II) fluoride nanorods. Near theoretical capacity (570 mA h g^-1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr_1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF₂.

High energy-density and reversibility of iron fluoride cathode enabled via an intercalation-extrusion reaction

Iron fluoride, an intercalation-conversion cathode for lithium ion batteries, promises a high theoretical energy density of 1922 Wh kg^–1_. However, poor electrochemical reversibility due to repeated breaking/reformation of metal fluoride bonds poses a grand challenge for its practical application. Here we report that both a high reversibility over 1000 cycles and a high capacity of 420 mAh g⁻¹ can be realized by concerted doping of cobalt and oxygen into iron fluoride. In the doped nanorods, an energy density of ~1000 Wh kg⁻¹ with a decay rate of 0.03% per cycle is achieved. The anion’s and cation’s co-substitutions thermodynamically reduce conversion reaction potential and shift the reaction from less-reversible intercalation-conversion reaction in iron fluoride to a highly reversible intercalation-extrusion reaction in doped material. The co-substitution strategy to tune the thermodynamic features of the reactions could be extended to other high energy conversion materials for improved performance.

Poor electrochemical reversibility of the conversion-type cathode materials remains an important challenge for their practical applications. Here, the authors report a highly reversible fluoride cathode material with low hysteresis through concerted doping of cobalt and oxygen into iron fluoride.

Atomistic Insights into FeF3 Nanosheet: An Ultrahigh-Rate and Long-Life Cathode Material for Li-Ion Batteries

Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF₃ results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, on the basis of first-principles calculations, first the stability of FeF₃ (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltages for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that (1) the formation of Li dimer is the most preferred, (2) the Li diffusion energy barrier of Li dimer is lower than isolated Li atom (0.17 eV for Li dimer vs 0.22 eV for Li atom), and (3) the diffusion coefficient of Li is 1.06 × 10^-6 cm²·s^-1, which is orders of magnitude greater than that of Li diffusion in bulk FeF₃ (10^-13-10^-11 cm²·s^-1). Thus, FeF₃ nanosheet can act as an ultrahigh-rate cathode material for Li-ion batteries. More importantly, the calculated voltage and specific capacity of Li on the FeF₃ (012) nanosheet demonstrate that it has a much more stable voltage profile than bulk FeF₃ for a wide range of Li concentration. So, few layers FeF₃ nanosheet provides the desired long-life energy density in Li-ion batteries. These above findings in the current study shed new light on the design of ultrahigh-rate and long-life FeF₃ cathode material for Li-ion batteries.

Ternary metal fluorides as high-energy cathodes with low cycling hysteresis

Transition metal fluorides are an appealing alternative to conventional intercalation compounds for use as cathodes in next-generation lithium batteries due to their extremely high capacity (3-4 times greater than the current state-of-the-art). However, issues related to reversibility, energy efficiency and kinetics prevent their practical application. Here we report on the synthesis, structural and electrochemical properties of ternary metal fluorides (M(1)yM(2)(1-y)F(x): M(1), M(2) = Fe, Cu), which may overcome these issues. By substituting Cu into the Fe lattice, forming the solid-solution Cu(y)Fe(1-y)F(2), reversible Cu and Fe redox reactions are achieved with surprisingly small hysteresis (<150 mV). This finding indicates that cation substitution may provide a new avenue for tailoring key electrochemical properties of conversion electrodes. Although the reversible capacity of Cu conversion fades rapidly, likely due to Cu(+) dissolution, the low hysteresis and high energy suggest that a Cu-based fluoride cathode remains an intriguing candidate for rechargeable lithium batteries.

The solid state conversion reaction of epitaxial FeF2(110) thin films with lithium studied by angle-resolved X-ray photoelectron spectroscopy

An epitaxial FeF₂(110) thin film was exposed to Li as a high-purity analogue of a Li-ion battery discharge. The stoichiometry and morphology of the film were then characterized by ARXPS.

Interplay between the ionic and electronic transport and its effects on the reaction pattern during the electrochemical conversion in an FeF2 nanoparticle

The continuous Fe network provides an electronic transport pathway, which in turn enables ionic transport through the interface.

Large-scale fabrication of graphene-wrapped FeF3 nanocrystals as cathode materials for lithium ion batteries

Graphene-wrapped FeF3 nanocrystals (FeF3/G) have been successfully fabricated for the first time by a vapour-solid method, which can be generalized to synthesize other metal fluorides. The as-synthesized FeF3/G delivers a charge capacity of 155, 113, and 73 mA h g(-1) at 104, 502, and 1040 mA g(-1) in turn, displaying superior rate capability to bare FeF3. Moreover, it exhibits stable cyclability over 100 cycles with a charge capacity of 185.6 and 119.8 mA h g(-1) at 20.8 and 208 mA g(-1), respectively, which could be ascribed to the buffering effect and lowered resistance from the graphene. This versatile vapour-solid method and the improved cyclability provide a promising avenue for the application of metal fluorides as cathode materials.